java - android:onclick method doesn't work -

xml code:

 <button             android:layout_width="match_parent"             android:layout_height="wrap_content"             android:text="invia"             android:clickable="true"             android:onclick="onclick1"             android:id="@+id/invia" /> 

java code:

import android.app.activity; import android.content.intent; import android.support.v7.app.appcompatactivity; import android.os.bundle; import android.view.view; import android.widget.button; import android.widget.edittext; import android.widget.toast;  import com.android.volley.request; import com.android.volley.requestqueue; import com.android.volley.response; import com.android.volley.volleyerror; import com.android.volley.toolbox.stringrequest; import com.android.volley.toolbox.volley;  import java.util.hashmap; import java.util.locale; import java.util.map;  public class creazionesondaggio extends appcompatactivity   {      string url;     private edittext titolo;     private edittext sceltaa;     private edittext sceltab;     private edittext sceltac;    private button invio;     private edittext server;     @override     protected void oncreate(bundle savedinstancestate)  {         titolo = (edittext) findviewbyid(r.id.titolo);         sceltaa = (edittext) findviewbyid(r.id.a);         sceltab = (edittext) findviewbyid(r.id.b);         sceltac = (edittext) findviewbyid(r.id.c);         server= (edittext) findviewbyid(r.id.server);        invio = (button) findviewbyid(r.id.invia);         // invio.setonclicklistener(this);          super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_creazione_sondaggio);     }      private void inviodati() {         ......     }      public void onclick1(view v) {         toast.maketext(creazionesondaggio.this, "ok", toast.length_long).show();             inviodati();        }   } 

error:

process: com.example.rober.registrazione, pid: 21261 java.lang.illegalstateexception: not execute method android:onclick @ android.support.v7.app.appcompatviewinflater$declaredonclicklistener.onclick(appcompatviewinflater.java:275) @ android.view.view.performclick(view.java:5210) @ android.view.view$performclick.run(view.java:21288) @ android.os.handler.handlecallback(handler.java:739) @ android.os.handler.dispatchmessage(handler.java:95) @ android.os.looper.loop(looper.java:148) @ android.app.activitythread.main(activitythread.java:5527) @ java.lang.reflect.method.invoke(native method) @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:730) @ com.android.internal.os.zygoteinit.main(zygoteinit.java:620) caused by: java.lang.reflect.invocationtargetexception @ java.lang.reflect.method.invoke(native method) @ android.support.v7.app.appcompatviewinflater$declaredonclicklistener.onclick(appcompatviewinflater.java:270) @ android.view.view.performclick(view.java:5210)  @ android.view.view$performclick.run(view.java:21288)  @ android.os.handler.handlecallback(handler.java:739)  @ android.os.handler.dispatchmessage(handler.java:95)  @ android.os.looper.loop(looper.java:148)  @ android.app.activitythread.main(activitythread.java:5527)  @ java.lang.reflect.method.invoke(native method)  @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:730)  @ com.android.internal.os.zygoteinit.main(zygoteinit.java:620)  caused by: java.lang.nullpointerexception: attempt invoke virtual method 'android.text.editable android.widget.edittext.gettext()' on null object reference @ com.example.rober.registrazione.creazionesondaggio.inviodati(creazionesondaggio.java:48) @ com.example.rober.registrazione.creazionesondaggio.onclick1(creazionesondaggio.java:87) @ java.lang.reflect.method.invoke(native method)  @ android.support.v7.app.appcompatviewinflater$declaredonclicklistener.onclick(appcompatviewinflater.java:270)  @ android.view.view.performclick(view.java:5210)  @ android.view.view$performclick.run(view.java:21288)  @ android.os.handler.handlecallback(handler.java:739)  @ android.os.handler.dispatchmessage(handler.java:95)  @ android.os.looper.loop(looper.java:148)  @ android.app.activitythread.main(activitythread.java:5527)  @ java.lang.reflect.method.invoke(native method)  @ com.android.internal.os.zygoteinit$methodandargscaller.run(zygoteinit.java:730)  @ com.android.internal.os.zygoteinit.main(zygoteinit.java:620)

how can fix it?

update oncreate method

 protected void oncreate(bundle savedinstancestate)  {   super.oncreate(savedinstancestate);         setcontentview(r.layout.activity_creazione_sondaggio);         titolo = (edittext) findviewbyid(r.id.titolo);         sceltaa = (edittext) findviewbyid(r.id.a);         sceltab = (edittext) findviewbyid(r.id.b);         sceltac = (edittext) findviewbyid(r.id.c);         server= (edittext) findviewbyid(r.id.server);        invio = (button) findviewbyid(r.id.invia);         // invio.setonclicklistener(this);       }